Distributions and sources of low-molecular-weight monocarboxylic acids in gas and particles from a deciduous broadleaf forest in northern Japan

Distributions of volatile monocarboxylic acids in a deciduous broadleaf forest

Distributions and sources of low-molecular-weight monocarboxylic acids in gas and particles from a deciduous broadleaf forest in northern JapanDistributions of volatile monocarboxylic acids in a deciduous broadleaf forestTomoki Mochizuki et al.

To better understand the distributions and sources of
low-molecular-weight (LMW) monocarboxylic acids (monoacids) in the forest
atmosphere, we conducted simultaneous collection of gaseous and particulate
samples at a deciduous broadleaf forest site in northern Japan. LMW normal
(C1–C10), branched (iC4–iC6), hydroxyl (glycolic and
lactic) and aromatic (benzoic) monoacids were detected in the gas and
particle phases. The dominant LMW monoacids in gas phase were formic (mean:
953 ng m−3) and acetic (528 ng m−3) acids followed by propionic
(37 ng m−3) or isopentanoic (42 ng m−3) acid. In the particle
phase, isopentanoic (159 ng m−3) was dominant, followed by acetic
(104 ng m−3) and formic (71 ng m−3) or lactic
(65 ng m−3) acids. Concentrations of LMW monoacids did not show
correlations with anthropogenic tracers such as nss-SO42- and
NO3-, indicating that anthropogenic contribution is not important.
Concentrations of C1–C6 monoacids in the gas phase showed positive
correlations (r2=0.21–0.91) with isobutyric acid (iC4), which
may be produced by microbial activity in soil. The forest soil may be a
source of gaseous C1–C6 monoacids in the forest atmosphere. Acetic
acid in the particle phase positively correlated with nonanoic acid (C9)
(r2=0.63), suggesting that formation of acetic and nonanoic acids is
associated with the oxidation of biogenic unsaturated fatty acids in the
aerosol phase, in addition to photochemical oxidation of biogenic volatile
organic compounds. The particle-phase fractions (Fp) of formic
and acetic acids showed negative correlation with ambient temperature
(C1: r2=0.49, C2: r2=0.60) but showed positive
correlation with relative humidity (C1: r2=0.30, C2: r2=0.55) in daytime, suggesting that these meteorological parameters are
important for the gas and particle portioning of monoacids in the forest
atmosphere.

Homologous series (C1–C10) of low-molecular-weight (LMW)
monocarboxylic acids (monoacids) are known to exist in the atmosphere as gas
and particle phases (e.g., Kawamura et al., 1985, 2000; Liu et al., 2012).
They have been reported from urban (Kawamura et al., 2000), forest (Andreae
et al., 1988), marine (Miyazaki et al., 2014; Boreddy et al., 2017) and
Antarctic samples (Legrand et al., 2004). Formic (C1) and acetic
(C2) acids are the dominant volatile organic species in the atmosphere. LMW
monoacids and their salts in aerosols are water soluble and thus can act as
cloud condensation nuclei (CCN), contributing to the Earth radiative forcing
directly or indirectly (Kanakidou et al., 2005) and affecting the radiation
budget of the atmosphere. Conversely, high abundances of LMW
monoacids in the troposphere can potentially have adverse effects on air
quality and human health and also increase the acidity of rainwater (Keene
et al., 1983; Kawamura et al., 1996).

LMW monoacids are directly emitted from fossil fuel combustion, biomass
and biofuel burning (Kawamura et al., 1985; Paulot et al., 2011), and
terrestrial vegetation (Kesselmeier et al., 1997; Jardine et al., 2011). In
addition, secondary production from photochemical oxidations of biogenic
volatile organic compounds (VOCs) such as isoprene and anthropogenic VOCs
such as acetylene and ethane is an important source of LMW monoacids (Paulot
et al., 2011). Recently, Stavrakou et al. (2012) conducted satellite
measurements of formic acid on a global scale. They suggest that boreal and
tropical forests are important sources of formic acid in the troposphere. In
the model experiment, Paulot et al. (2011) estimated that the global sources of
formic and acetic acids are ∼1200 and
∼1400 Gmol yr−1, respectively; however, these values
are highly uncertain.

In our previous study, normal (C1–C10), branched
(iC4–iC6) and hydroxy (glycolic and lactic) monoacids were
detected in gas, aerosol and snow pit samples (Kawamura et al., 2000;
Mochizuki et al., 2016, 2017). In particular, branched (iC5) and hydroxy
(lactic) monoacids were abundantly detected in aerosol samples from northeast
China (Mochizuki et al., 2017). Detected branched (iC4 and iC5) and
hydroxy (lactic) monoacids are likely derived from microorganisms and plants
(Curl, 1982; Effmert et al., 2012). However, those monoacids have not been
reported in the forest atmosphere, in which an ion chromatograph was used,
and the species detected are generally limited to formic and acetic acids
(Tsai and Kuo, 2013). Because LMW monoacids including hydroxy acids are
highly water soluble, they can alter the hygroscopic properties of
atmospheric particles. There is no study on gas–particle partitioning of
normal (C1–C10), branched (iC4–iC6) and hydroxy
(glycolic and lactic) monoacids in the forest atmosphere. Therefore, the
study of LMW monoacids in forest is important.

In this study, we collected gas and particle samples from a deciduous
broadleaf forest of northern Japan in summer. To better understand the
distributions and sources of LMW monoacids, the samples were analyzed for
normal (C1–C10), branched (iC4–iC6), hydroxyl
(glycolic and lactic) and aromatic (benzoic) monoacids in both gas and
particle phases using capillary gas chromatography. Inorganic ions were
measured in the particle phase. We discuss the importance of monoacid-enriched
aerosols and their possible sources in the forest atmosphere.

The Sapporo forest meteorology research site (SAP) (42∘59′ N,
141∘23′ E, 182 m a.s.l.) is located in a hilly area (147 ha)
neighboring the urban district of Sapporo, Hokkaido, Japan (Fig. 1).
Residential areas are located north, east and west of the site. The forest
type is matured secondary deciduous broadleaf forest. The major tree is
Japanese white birch (Betula platyphylla var. japonica) and
Japanese oak (Quercus mongolica var. grosse serrata). The
major understory is a dwarf bamboo (Sasa kurilensis and Sasa senanensis). Metrological data such as ambient temperature, relative
humidity (RH), UV-A, wind speed, wind direction and precipitation were taken
at a meteorological tower (Fig. 2). Details of the micrometeorological
measurements and site information have been described in Yamanoi et
al. (2015) and Miyazaki et al. (2012a, b). During the campaign period (June
to July 2010), ambient air temperature ranged from 18 to 26 ∘C
(average: 21±2.3∘C), whereas RH ranged from 69 % to
96 % (average: 87±7.9 %). UV-A was high during the first half of
the campaign (except for 7 July) and low during the second half. The dominant
wind direction throughout the campaign was from the east and south. Wind
speed ranged from 0.2 to 0.6 m s−1 (average: 0.4 m s−1).
Precipitation occurred in the morning of 1 July (11 mm), in the evening of 4
July (1.2 mm), and in the morning of 8 July (6.6 mm).

Samplings were conducted from 28 June to 8 July 2010. The samples were
collected for 15 h (05:00–20:00 LT) in daytime (n=11) and 9 h
(20:00–05:00 LT) in nighttime (n=11). Total suspended particles (TSPs) and
gaseous organic acids were collected using a low-volume air sampler equipped
with two-stage filter packs (URG-2000-30FG) at a flow rate of 15 L min−1 (Kawamura et al., 1985). The particles were collected onto
precombusted (450 ∘C, 6 h) quartz-fiber filters (47 mm
diameter) (first stage), whereas gaseous organic acids were collected on the
quartz-fiber filter impregnated with potassium hydroxide (KOH) (second
stage). The KOH-impregnated filters were prepared by rinsing the
precombusted quartz filter in a 0.2 M KOH solution and then dried in an oven
at 80 ∘C. Each filter was placed in a clean glass bottle with a
Teflon-lined screw cap. After the sampling, the filter samples were stored
in a freezer room at −20∘C prior to analysis. Semi-volatile
organic acids collected on the first filter may in part evaporate, causing
negative artifacts. Conversely, the second filter may adsorb organic
vapors evaporated from the first filter, causing positive artifacts.
Although such artifacts are possible for any filter-based measurements under
ambient conditions, these effects are minimal (Kawamura et al., 1985).

LMW monoacids were determined as p-bromophenacyl esters using a capillary
gas chromatograph equipped with a flame ionization detector (GC–FID) and a
GC with a mass spectrometer (GC–MS) (Kawamura and Kaplan, 1984; Mochizuki et
al., 2017). Briefly, an aliquot of filter (4.3 cm2) was extracted for
water-soluble organic compounds with organic-free ultrapure water
(resistivity of > 18.2 MΩ cm) under ultrasonication. To
remove the particles, water extracts were filtered through a Pasteur pipette
packed with quartz wool. The pH of filtrates was adjusted to 8.5–9.0 with
0.05 M KOH solution. The samples were concentrated down to 0.5 mL using a
rotary evaporator under a vacuum at 50 ∘C. The concentrates were
passed through a Pasteur pipette packed with a cation exchange resin (DOWEX
50WX4, 100–200 mesh, K+ form). Free monocarboxylic acids were
converted to organic acid salts (RCOO-K+). After confirming the pH
of 8.5–9.0, the samples were dried using a rotary evaporator under vacuum at
50 ∘C, followed by blowdown with pure nitrogen gas. Organic acids
were derivatized to p-bromophenacyl esters in acetonitrile (4 mL) with
α,p-dibromoacetophenone (0.1 M, 50 µL) as a
derivatization reagent and dicyclohexyl-18-crown-6 (0.01 M, 50 µL) as a catalyst at 80 ∘C for 2 h (Kawamura and Kaplan, 1984). In
addition, OH functional groups in p-bromophenacyl esters of hydroxy
monoacids were reacted with
N,O-bis-(trimethylsilyl)trifluoroacetamide (BSTFA) with 1 %
trimethylsilyl chloride and 10 µL of pyridine at 70 ∘C
for 3 h to derive trimethylsilyl (TMS) ethers of p-bromophenacyl esters
(Kawamura et al., 2012).

p-Bromophenacyl esters and their TMS ethers were identified and quantified
using a capillary gas chromatograph (HP GC 6890, Hewlett-Packard, USA)
equipped with a flame ionization detector and GC–MS (Agilent 7890A and 5975C MSD, Agilent, USA). Details of the methods have been
described in Kawamura and Kaplan (1984) and Kawamura et al. (2012).
Recoveries of authentic monoacids (C1–C10, iC4–iC6,
glycolic, lactic and benzoic acids) spiked to a quartz filter were better
than 80 %. Analytical errors using authentic monoacids were within 12 %.

To measure inorganic ions, a portion of quartz-fiber filter (first stage)
was extracted with ultrapure water under ultrasonication. The extracts were
passed through a membrane disk filter (0.22 µm, Millipore Millex-GV,
Merck, USA). The filtrates were injected into an ion chromatograph (model
761 compact IC, Metrohm, Switzerland) (Boreddy and Kawamura, 2015). We
measured cations (Na+, NH4+, K+, Mg2+ and
Ca2+) and anions (F−, MSA−, Cl−, NO2-,
NO3-, PO4- and SO42-) in aerosol samples.
Concentrations of non-sea-salt SO42- [nss-SO42-] is
calculated with the following equation:

[nss-SO42-]=[SO42-]-0.25×[Na+],

where [SO42-] and [Na+] are concentrations of total
SO42- and Na+, respectively (Berg Jr. and Winchester,
1978; Duce et al., 1983).

We calculated 7-day air mass back trajectories at a height of 300 m a.s.l.
using the Meteorological Data Explorer (METEX) provided by the National
Institute for Environmental Studies
(http://db.cger.nies.go.jp/metex/index.html, last access: 12 August
2018).

Figure 4 shows day–night variations in selected monoacids in the gas and
particle phases. Gaseous formic acid did not show any day–night trend,
whereas particulate formic acid showed a diurnal distribution with higher
concentrations in the nighttime than daytime. Abundances of gaseous acetic acid
were higher in the daytime than in the nighttime, whereas an opposite trend was
found for acetic acid in the particle phase; that is, particulate acetic acid
was more abundant in the nighttime than in the daytime, being similar to particulate
formic acid. Day-to-day variations in other monoacids did not show a clear
diurnal trend.

Figure 4Diurnal variations in the concentrations of major monocarboxylic
acids in the gas (open circle) and particle (solid diamond) phases and inorganic
ions (nss-SO42- and NH4+). Day: D; night: N.

The highest concentrations of isopentanoic and lactic acids in the particle
phase were observed at night on 30 August. Temporal variation in
isopentanoic acid in particle phase was similar to that of lactic acid. Gas
and particulate concentrations of isopentanoic and lactic acids did not show
any clear diurnal variation.

The particle-phase fractions (Fp) of individual monoacids were
calculated as Fp=P/(G+P), where P is particle-phase concentration
and G is gas-phase concentration. Table 1 summarizes mean Fp of
individual monoacids in the daytime and nighttime in the deciduous broadleaf
forest. Fp of individual monoacids ranged from 0.04 (C3) to 0.63
(iC5) in the daytime and 0.05 (C3) to 0.69 (iC5) in the nighttime.
Formic (C1) and acetic (C2) acids are largely present in the gas
phase. Nonanoic (C9) and decanoic (C10) acids are present not only
in the gas phase but also in the particle phase. Glycolic and lactic acids are
largely present in the aerosol phase in the forest atmosphere (Table 1).

We detected cations (Na+, NH4+, K+, Mg2+ and
Ca2+) and anions (NO3-, SO42-, MSA−, Cl−,
NO2- and F−) in particle samples from a deciduous broadleaf
forest. Nss-SO42- (mean: 2240 ng m−3) is a major anion and
NH4+ (mean: 972 ng m−3) is a major cation. Concentrations of
major inorganic ions did not show clear diurnal or temporal variations. The pH
of the water extracts from particle samples ranged from 3.5 to 6.3 (mean:
5.0). We found that particle samples were always acidic in this study.

This forest site is located a few kilometers south of the Sapporo metropolitan
area. As discussed in the next section, lifetimes of monoacids are
relatively long (e.g., 12.9 days for formic acid), suggesting long-range
atmospheric transport of monoacids from other areas. The dominant wind
direction was from the east and south throughout the sampling period. We
compared the concentrations of individual monoacids together with
nss-SO42- and NO3-: anthropogenic tracers to evaluate
the influence of anthropogenic air mass transport from urban areas. We
confirmed that individual monoacids in both the gas and particle phases did not
show any significant correlations with nss-SO42- (r2 < 0.14) and NO3- (r2 < 0.11). The majority of
sampled air was not influenced by urban air masses. In addition, Fig. 5
shows 7-day air mass back trajectories (300 m a.s.l.) for the study
period from 28 June to 8 July at the sampling site. Most of the air masses
passed through the Pacific Ocean during the measurement period, except for
28 June. This result may suggest that the air masses arriving at the forest
site are not affected by the outflows from East Asia and Far East Russia.

Figure 5The 7-day air mass back trajectories at a height of 300 m a.s.l.
during the sampling period.

4.1 Possible sources of LMW monoacids

To better understand molecular distributions of monoacids in the gas phase
(i.e., predominance of formic acid followed by acetic acid), we calculated
the lifetimes of gaseous C1–C4 and iC4 monoacids with OH
radicals (OH radical concentration =2.0×106 molecule cm−3) using the
rate constants of gaseous C1–C4 and iC4
monoacids (provided by NIST Chemical Kinetics Database). The lifetimes of
gaseous formic, acetic, propionic, butyric and isobutyric acids with OH
radicals are 12.9, 8.6, 4.8, 3.2 and 2.8 days. These results showed that
organic acids are relatively stable with longer lifetimes for shorter-chain
monoacids. This unique feature of lifetime can explain the predominance of
formic acid due to the accumulation in the gas phase and high concentrations of
formic and acetic acids in the atmosphere.

LMW monoacids are directly emitted from fossil fuel combustion (Kawamura et
al., 1985) and plant leaves (Kesselmeier and Staudt, 1999) and also produced
in the atmosphere by photooxidation of anthropogenic and biogenic VOCs
(Paulot et al., 2011). LMW monoacids have a variety of anthropogenic and
biogenic sources. In the gas phase, isobutyric acid (iC4) showed positive
correlations with C1 (day: r2=0.36, night: no correlation),
C2 (0.53, 0.43), C3 (0.76, 0.64), C4 (0.82, 0.80), C5
(0.91, 0.81) and C6 (0.72, 0.74) monoacids (Fig. 6). Branched chain
monoacids including isobutylic acid are known as common metabolites of
bacteria (e.g., Bacteroides distasonis) and fungi in soils (Effmert et al., 2012, and references
therein). Correlations of C1–C6 monoacids with iC4 suggest
that forest floor is a source of gaseous C1–C6 monoacids in the
forest atmosphere. LMW monoacids such as acetic and propionic acids can be
produced by microbiological processes (Effmert et al., 2012). In addition,
exudation of organic acids is known to occur in vascular plants, mainly from
roots (Curl, 1982). Shen et al. (1996) reported that formic, acetic and
propionic acids are contained in forest soil and rhizosphere soil.

Figure 6Concentrations of C1–C6 monoacids against isobutyric
acid (iC4) in the gas phase. The coefficient of determination shows that
the regression line is statistically significant (p < 0.05).

Although we did not collect a forest soil sample from Sapporo during the
air-sampling period, we collected a surrogate soil sample (surface ∼3 cm) from a broadleaf forest at Chubu University campus in central Japan
on 31 October 2018. The forest floor at the Chubu University site is similar
to that of the Sapporo site in terms of the coverage with a broadleaf litter
from similar plant species including a Japanese oak. The climate in central
Japan is different from northern Japan, but the air temperature recorded in
Nagoya next to the Chubu University campus in October 2018 (average: 19±2.9∘C; Japan Meteorological Agency:
https://www.jma.go.jp/jma/indexe.html, last access: 31 January 2019)
was similar to that of Sapporo (21±2.3∘C) during the
air-sampling period. These similarities provide a strong justification to
utilize the soil sample from the Chubu University site as a surrogate of
Sapporo forest soil. The soil sample was analyzed for LMW monoacids after
water extraction employing the analytical protocol described in the
experimental section. LMW normal (C1–C10), branched (iC4) and
hydroxyl monoacids were detected in the soil sample (Kunwar et al.,
unpublished data, 2018). We found high abundances of formic
(7400 ng gwetsoil-1) and acetic
(4260 ng gwetsoil-1) acids, which are significantly
higher than the rest of the monoacids (∼1800 ng gwetsoil-1). Interestingly, hydroxy acids such as glycolic
(1680 ng gwetsoil-1) and lactic
(1860 ng gwetsoil-1) acids were abundantly detected in
the soil sample together with isobutyric acid (77 ng gwetsoil-1) (Kunwar et al., unpublished data, 2018). These preliminary
results suggest that monoacids in the forest atmosphere are in part derived
from forest soil via microbial decomposition of plant debris and subsequent
emission to the air.

However, it is not easy to estimate the quantitative contribution of
monoacids from the forest floor. It is likely that molecular composition of
LMW monoacids in soil may depend on a variety of parameters including types
of microorganisms in soil, soil organic matter and exudation from plant
roots. Conversely, we consider photooxidation of biogenic VOCs
such as isoprene and monoterpenes to be an important source of formic and
acetic acids in the atmosphere (Paulot et al., 2011).

In the particle phase, a positive correlation was observed between acetic acid
and nonanoic acid (day: r2=0.63, night: r2=0.63) (Fig. 7). Unsaturated fatty acids (UFAs) such as oleic (FA18:1) and linoleic
(FA18:2) acids are generally present in terrestrially higher plants and
soil fungi (Yokouchi and Ambe, 1986; Kaur et al., 2005). Nonanoic (C9)
and hexanoic (C6) acids are produced by the heterogenous oxidation of
FA18:1 and FA18:2 in aerosols, respectively, via the cleavage of a
double bond at the C9 position of UFAs (Yokouchi and Ambe, 1986; Kawamura
and Gagosian, 1987). Longer-chain monoacids may produce acetic acid via
photochemical breakdown with OH radicals. UFAs may also contribute to the
formation of acetic acid in aerosol in a deciduous broadleaf forest.

Figure 7Concentrations of acetic acid in the particle phase as a function of
those of nonanoic acid. The coefficient of determination shows that the
regression line is statistically significant (p < 0.05).

Relatively high abundances of particulate lactic and isopentanoic acids were
observed in the forest atmosphere (Table 1). A positive correlation was
observed between lactic acid and isopentanoic acid in the particle phase
(r2=0.98). Particulate lactic acid did not show correlations with
other LMW monoacids detected in the particle phase (r2 < 0.17).
Isopentanoic acid can be produced by bacteria Clostridium spp. and Bacteroides spp. (Effmert et
al., 2012, and references therein). We confirmed that lactic acid is
abundantly present in the forest soil from central Japan (1860 ng gwetsoil-1), but isopentanoic acid is below the detection limit (Kunwar
et al., unpublished data, 2018). Lactic acid is produced not only by
bacteria (lactobacillus) (Cabredo et al., 2009) but also by the oxidation of isoprene
with ozone (Nguyen et al., 2010). The microflora community in soil system may be
different between the two sites: a soil-sampling site in central Japan and
an air-sampling site in northern Japan. More in-depth studies are needed to
better understand the soil-to-air emissions of normal, branched and hydroxyl
monoacids from the forest floor and the subsequent interaction between soil
and the overlying atmosphere.

Formic and acetic acids in the particle phase show clear day–night variations
with higher concentrations in the nighttime than in the daytime. The higher
concentrations in the nighttime may be associated with a shallower planetary
boundary layer, which can accumulate organic acids near the ground surface.
It is important to note that wind speed in the nighttime (average: 0.3 m s−1) was comparable to that in the daytime (average: 0.4 m s−1) and no
correlation was observed between the concentrations of formic and acetic
acids and wind speed (r2 < 0.01), suggesting an importance of
other meteorological parameters that control the gas–particle portioning of
organic acids.

We investigated the effects of ambient temperature on gas–particle
partitioning of LMW monoacids. Fp values of formic and acetic acids were found
to decrease with an increasing ambient temperature (C1: r2=0.49; C2: r2=0.60) (Fig. 8), whereas other LMW monoacids did
not show clear correlations with ambient temperature (r2 < 0.37), except for butyric acid (r2=0.70) in the daytime. Although
Fp values of LMW monoacids did not show a significant correlation with ambient
temperature in the nighttime (r2 < 0.16), except for propionic acid
(r2=0.31), we found that average Fp values of LMW monoacids in
the nighttime were higher than those in the daytime (Table 1). A higher temperature
promotes the transfer of aerosol-phase formic and acetic acids to the gas phase
in the daytime by evaporation, which is consistent with Henry's law constants.
Khan et al. (1995) reported that ambient temperature is an important factor
to control the gas–particle partitioning of organic acids.

Figure 8Particle-phase fractions (Fp) of formic and acetic acids
against temperature. The coefficient of determination shows that the regression
line is statistically significant (p < 0.05).

Conversely, we found that Fp values of formic and acetic acids increase
with an increasing RH in the daytime (C1: r2=0.30; C2:
r2=0.55) (Fig. 9), whereas other LMW monoacids did not show a
significant correlation with RH (r2 < 0.20), except for butyric
acid (r2=0.55). In the nighttime, Fp values of LMW monoacids did not show
any significant correlation with RH (r2 < 0.15). Al-Hosney et al. (2005) and Prince et al. (2008) reported that the uptake of formic and
acetic acids by CaCO3 can be enhanced by a higher RH (C1: RH > 62 %; C2: RH > 53 %). In this study, we
estimated liquid water contents (LWCs) of aerosols using the ISORROPIA II model
(Fountoukis and Nenes, 2007), the data of inorganic ions and meteorological
parameters. The estimated aerosol LWCs ranged from 1.4 to 14.6 µg m−3 (av. 6.4 µg m−3). Although Fp values of LMW monoacids did
not show any strong correlations with LWC (r2 < 0.24), strong
positive correlations were found between RH and aerosol LWC in the daytime
(r2=0.47) and in the nighttime (r2=0.74). A higher RH may
enhance the transfer of gaseous formic and acetic acids to the aerosol phase as
a result of the condensation of water vapor on aerosol particles.

Figure 9Particle-phase fractions (Fp) of formic and acetic acids
against relative humidity. The coefficient of determination shows that
the regression line is statistically significant (p < 0.1).

The larger Fp values were obtained for lactic acid (daytime:
0.60;
nighttime: 0.69), although we obtained fewer smaller values for glycolic
acid (0.47–0.48, Table 1). These levels are comparable to those reported for
the Pacific Ocean (Fp=0.82) (Miyazaki et al., 2014). Although the
vapor pressure of lactic acid (5.3×10-4 atm) is higher than
that of C5–C10 monoacids (vapor pressure: 1.6×10-4–1.6×10-10 atm), the Fp of lactic acid was
larger than that of C5–C10 monoacids (Fp: 0.11–0.50). This
apparent discrepancy may suggest the possibility that lactic acid is partly
present as bioaerosols such as bacterial particles possibly emitted from soil
surface in the forest atmosphere.

Gaseous organic acids react with alkaline particles such as calcium
carbonate, promoting the gas–particle partitioning of organic acids
(Alexander et al., 2015). We calculated total cation equivalents (Na+,
NH4+, K+, Mg2+ and Ca2+) minus total anion
equivalents (F−, MSA−, Cl−, NO2-, NO3-
and SO42-) including monoacids detected, although CO32-,
HCO3- and unidentified organic anions were not considered. Total
cations were higher than total anions. No positive correlations were
observed (r2 < 0.04) between Fp of individual LMW monoacids
and excess cations. This result indicates that excess cations are not an
important factor to control the gas–particle partitioning of LMW monoacids
in the forest atmosphere. Fp values of LMW monoacids showed positive
correlations with mass concentrations of total LMW monoacids in the particle
phase (r2=0.24–0.46), except for C3, C9 and C10.
This result suggests that gaseous LMW monoacids may be adsorbed on the
preexisting particles in the forest atmosphere, although we did not measure
the aerosol mass concentrations.

We conducted simultaneous sampling of gaseous (G) and particulate (P) LMW
monoacids in a deciduous broadleaf forest from northern Japan, followed by
gas chromatographic determination after p-bromophenacyl ester
derivatization of monoacids. LMW normal (C1–C10), branched chain
(iC4–iC6), hydroxyl (glycolic and lactic) and aromatic (benzoic)
acids were detected in both gas and aerosol phases. Formic and acetic acids
were found as the dominant species followed by propionic acid in the gas
phase, whereas isopentanoic, acetic and formic acids were detected as major
monoacids in the particle phase. Particle-phase fractions (Fp=P/(P+G)) of major LMW monoacids were low (Fp: 0.04-0.32),
although nonanoic (Fp=0.50), decanoic (0.43), isopentanoic
(0.68) and lactic (0.66) acids are largely present in the aerosol phase.
Concentrations of C1–C6 monoacids in the gas phase showed positive
correlations with isobutyric acid (iC4) (r2=0.21–0.91). Branched
chain monoacids that are common metabolites of bacteria and fungi may be
derived from the microbial degradation of leaf and plant debris in the forest
soil. In addition to atmospheric oxidation of VOCs and UFAs as an important
source of organic acids, we suggest that the forest floor is another source
of gaseous LMW monoacids in the forest atmosphere. Acetic acid in the
particle phase showed a positive correlation with nonanoic acid (C9),
which is produced by the oxidation of unsaturated fatty acids such as oleic
acid. Fp of formic and acetic acids showed negative correlations
with ambient temperature (C1: r2=0.49; C2: r2=0.60)
and positive correlations with RH (C1: r2=0.30; C2: r2=0.55) in the daytime, suggesting that these meteorological parameters are
important factors to control the gas–particle portioning of LMW monoacids in
the forest atmosphere. The present study demonstrates that deciduous
broadleaf forest is an important source of LMW monoacids in the gas and
particle phases in the atmosphere of northern Japan.

This study was in part supported by JSPS KAKENHI grant numbers JP19204055 and
JP18K18181. We thank Katsumi Yamanoi and Yasuko Mizoguchi of the Forestry and
Forest Products Research Institute for the courtesy of using the sampling
site.